Hollow fiber semipermeable membranes are presently being used extensively in a wide range of diffusive and/or convective separation processes. More particularly the hollow fiber membranes to which the present invention is particularly directed are used, although not exclusively, to quite a considerable extent in connection with the extracorporeal treatment of blood, such as in hemofiltration, hemodialysis and the like.
It is well known to produce hollow fiber membranes by a technique which includes the coextrusion of a polymer dissolved in a suitable solvent or solvent system along with a fluid which may be inert, or which may act as a coagulation or precipitation fluid. In these techniques, the polymer solution is generally extruded through a circular orifice, and the inert fluid or coagulating or precipitating fluid is extruded out of a central bore within the center of the circular slit through which the polymer is being extruded. This fluid is thus called the center fluid or center medium. It is also common practice to then allow the coextrudate to pass through the atmosphere for a short time, generally ranging from fractions of a second up to several seconds, and in many cases the length of that air gap is said to influence the properties of the resulting hollow fiber membrane. Further, it is also common in these processes to draw the coextrudate through various fluids which may be, but are not necessarily, identical with the center fluid. The center fluid and/or these consecutive treatment fluids cause a phase inversion, and finally precipitation and solidification of the membrane. These processes are thus decisive of the various membrane properties, such as those of the pore structure, pore size, pore size distribution, wettability, burst pressure, permeability and rejection properties, etc. thereof.
One property of such hollow fiber membranes which is of extreme importance, particularly in connection with the use of such hollow fiber membranes in medical applications, is the surface energy. The surface energy of a polymer depends on the nature of those groups which are present on its surface. In the case of blood contact, it is well known that a surface which has a high surface energy tends to exhibit a lower tendency to adsorb proteins or cellular components that may cause coagulation, as compared to such surfaces which have low surface energies. Furthermore, these surface energies correspond to the wettability of these surfaces. That is, the higher the surface energy of such a surface, the better will be its wettability. Water is generally employed as the wetting agent. Thus, the lower the contact angle with water, the higher is both the wettability and the surface energy thereof.
Membranes which have highly wettable surfaces will also tend to have high diffusive permeabilities, and therefore to have higher clearances when used in dialysis processes and the like. An object of this invention is therefore to provide a process for producing hollow fiber membranes with an inner fiber surface having improved wettability characteristics, which in turn leads to higher performance and reduced fouling thereof.
It is also a common technique to collect these hollow fiber membranes into bundles, and to then pot them into suitable housings. The most widely used material is two component polyurethane, and for potting these bundles of membranes into this type of potting material it is essential to first bring the hollow fiber membranes into a dry state. Therefore, these hollow fiber membranes must be stabilized against irreversible shrinkage and loss of filtration properties during drying. This is commonly done with glycerol, especially with membranes intended for medical use, as is disclosed, for example, in European Patent Publication No. 0 046 817.
The most common technique presently being employed requires a series of discontinuous production steps between precipitation of the hollow fiber membrane and the preparation of a bundle which is ready to be potted. In particular, the hollow fiber membranes must be collected, made into bundles, the center fluid has to be removed, the hollow fiber membranes must be rinsed, treated with a stabilizing agent against shrinkage and loss of properties during drying, excess stabilizing agent must be removed, and the bundle of hollow fiber membranes must then be dried in order to be ready for potting.
There is obviously significant interest in finding a production procedure which avoids the use of expensive rinsing procedures and which reduces the need to rely on these discontinuous production steps. Another object of this invention is therefore to provide a process for producing such dry, hollow fiber membrane bundles which are ready for potting on line.